Let’s say you run across a lightweight, inexpensive, easy-to-manufacture metal that you are considering for a new bike frame. Beyond the functional considerations of strength, size, and finish options what else do you consider?
Is it durable? If it fails, how does it fail (e.g., a shattering of a bicycle frame would not be good). You may also consider how the bicycle will be used and stored. What stress will the frame experience over its lifetime?
Having a single document that tablulates the range of use conditons and stress distribution would fully answer these intial questions. Furthermore, a environmental test manul would outline how to evaluate a design’s ability to have adequate design margin to accommodate the expected stresses.
Stress versus Strength
The concept of stress–strength curves come to mind. At the moment you have the material’s datasheet, which describes tensile and bend strength plus a range of other material properties. Once the bicycle frame is formed there is a range of tests to conduct to determine how the system once built will perform. Basically, these tests will help estimate the strength curve.
What about the stress curve? Where does that come from? How strong is strong enough? If the frame is able to withstand 100 shock loads similar to hitting an 8-inch-high concrete curb, is that good or not? How many times will riders in this market strike a curb over the life a bike?
The process of estimating or determining the range of loads is different than evaluating the strength part of the stress–strength relationship. We need to consider the rider’s behavior plus the environment.
Creating Stress Curves
For a specific model of bicycle typically used by a specific type of rider, we may have very different sets of stress conditions and behaviors. A child’s first bike may experience very low power exerted to the pedals, maybe 10 to 100 W; in contrast, a world-class competitive rider may impart close to 1,000 W at times. The force across the frame is thus dramatically different.
A combination of observations, measurements, simulations, and studies allows your team to build a set of expected stress conditions for different classes of products. The weight, frequency of riding, distance traveled, and the many other factors that shape the set of stress conditions are different for each family of bicycles.
One approach is to understand the limits of expected stress and design a product to always meet those limits. Designing a child’s bicycle based on the competitive rider’s set of stresses is not practical. Designing, building, and testing a product to meet the customer’s set of stress conditions is a common approach.
Collecting the Stress Conditions
Each bicycle frame has a target rider in mind: an avatar or typical user of the specific style of bicycle. At first the team may make some educated guesses about the behavior of typical riders and where and when they ride or store the bicycle. Over time and with some investment you gather information about the ranges of specific stresses that impact the performance and durability of the bicycle family of designs: the minimum and maximum and the distribution of temperatures during riding and storage, for example.
The list of stresses that apply may include weather-related conditions, such as temperature, humidity, rain, snow, or ice, as well as plus conditions related to use, such as shocks from street curbs, drops, or transport on a car rack. Stress also varies by the style of riding. For example, a recreational rider may only lightly stress the mechanical loading of the frame, yet a professional rider will notice the slightest change in flex across the frame as it impact power transfer and handling.
Gather the data related to performance, durability, and reliability. Identify what sets of stress may incrementally damage the materials, embrittle metals or welds, fade paint, or impede smooth operation. Characterize the stress with minimum, maximum, and nominal values. Also, work to determine the distribution of values across the range (the stress curve).
Benefits of an Environmental Manual
The environmental test manual tabulates and describes the stress curves for the various types of riders for each family of products. The document provides a repository for the conditions the product may experience during transport during the sales process or during use, plus storage and use conditions.
The testing focuses on measuring the strength curve related to a specific stress curve. This enables conclusions to be drawn. Is the strength of this design sufficient to withstand the expected stress curve initially and over time?
An added benefit of tabulating the stress curves is that it provides the entire team with a common set of conditions that are clearly spelled out and supported. It helps avoid the issue in which one designer believes the design will be strong enough when mistakenly considering only the average weight of riders and not the proportion of heavier riders.
The benefit of having distributions rather than limits alone is the ability to balance the expected failure rates with the proportion of customers who may experience failures. Although it may be uneconomical to create a recreational bicycle frame that performs well for a person on the maximum power (force on the pedals) end of the scale, there are not many 500+ W (high sustained power) bicycle riders in the recreational market.
When serving the same market, consider that each product is not unique. Relearning or discovering the set of expected stresses is not necessary. Create an environmental profile and the associated strength testing for each distinct market. As materials, processes, and markets change, update the manual.
An environmental test manual is like an internal standard for your products based on your understanding of your customers. Publicly available standards provide a starting point, yet they will not provide sufficient insight and information for your product in the hands of your customers.
Bio:
Fred Schenkelberg is an experienced reliability engineering and management consultant with his firm FMS Reliability. His passion is working with teams to create cost-effective reliability programs that solve problems, create durable and reliable products, increase customer satisfaction, and reduce warranty costs. If you enjoyed this articles consider subscribing to the ongoing series at Accendo Reliability.